So if USB stands for universal serial bus, that, to me, means there is only one stream of charge, however some cables are significantly faster than others. Is this due to the width of the cable? Or quality of the material in the cable?
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USB cables contain power lines and signal lines. Some very cheap cables don’t contain signal lines, so the charger and device can’t talk to each other and negotiate the supply power. In addition to this, for higher power levels the cables contain a chip, this signals to the attached devices how much current and voltage can safely flow through the cable.
Without this chip, charging will default to 5v 1.5A (or lower).
The cables themselves usually aren’t what makes the difference.
There is some impact of the resistance due to length, diameter and material of connectors of cables, but that is not really the big thing.
What usually makes the difference is the small box between the wall outlet and the cable.
The original USB Standard didn’t anticipate having to power much more than for example the indicator lights on a keyboard. Most computer peripherals in those days either had their own power connectors or did not need much power at all.
Once the option of providing a bit of power over USB was there engineers too advantage of it. It became the default way to power small electronics that didn’t need much power.
However moder phones can draw much more power than was described in the original standard. To do this via USB manufacturers have expanded the original standard so the device and the charger talk to one another about how much they both can do.
Phones can still charge at the old default rate without that it will just take longer.
Modern versions of USB have higher power draw build in the standard, but that only works if both sides can do it.
Imagine trying to breathe through a straw. The thinner it is, the less air you get. Electrical resistance works similar to that in that you lose voltage across the cable if the wires are too thin, too long, or made from aluminum instead of copper.
That’s a problem with USB cables because the standard USB voltage (5V) happens to be very close to the voltage that’s needed to charge a lithium-ion battery, and if you lose just a little bit to cable resistance, it will have a major impact on the charging speed.
Resistance: longer cables and/or those with thinner wires will have a higher resistance, which may limit the power that the device can charge at. Also, copper has a lower resistivity than aluminum, but is more expensive.
There are also some cheaply made cables that lack any conductors for the data pins, which may prevent the device and host from recognizing that they are supposed to be a charging setup, preventing higher currents from being drawn.
None of the answers given by the other commenters are the main reason. It is because USB does not actually refer to only one type. USB refers to many standards that have developed over the last 20 years. As the standards have improved (1 to 2 to 3 / 3.1 to 3.2 to 4) the data transfer rates and power delivery have increased. However for this to work, they needed to utilise more pins in the ports among other things. So in reality these cables are built different based on the USB standard they are designed for. Plugging a USB 2 cable into a USB 3 port will limit you to USB 2 speeds.
Edit furthermore there are standards in USB 3 called power delivery that can deliver up to 100W, so it very much depends on the design of the cable, and the port it is plugged into. (This is why you can buy cables that are specialised for data transfer or for lower delivery.)
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